Telephone system



4. Sheets-Sheet 1 INVENTOR By I L. WRIHT 0H9 Gm ATTORNEY P.- L. WRIGHTAug. 2%, 1946.

Aug. 2%, 343. wRlGHT ZAQ, TELEPHONE SYSTEM Filed Sept. 2, 1944 4Sheets-Sheet 2 INVENTOR P. L. WRIGHT I By $42M A T TORNE V MAI/V PRIVATEBRANCH EXCHANGE Aug. 20, 1946. v v p, wm 2,4@6,053

TELEPHONE SYSTEM Filed-se ta, 1944 4 Sheets-Sheetfi INVENTOR RAM RIGHT-A TTORNEY- Aug. 20, 1946. 'P. L. WRIGHT TELEPHONE SYSTEM Filed Sept;2,1944 4 Sheets-Sheet 4 mkommmfih w #3 mekwmumo a INVENTOR .T m Ma Ma w.M m T A Patented Aug. 20, 1946 TELEPHONE SYSTEM Paul L. Wright, Newark,N. J., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application September 2, 1944,Serial No. 552,403

Claims.

This invention relates to telephone systems and particularly to systemscomprising automatic switching apparatus selectively controlled bysubscribers and operators to establish desired connections.

Objects of the invention are the provision and improvement oftransmission control in semiautomatic telephone systems and particularlythe provision of a trunk circuit arranged for decreasing the attenuationof speech currents transmitted therethrough on connections which includehigh loss lines or trunks.

According to this invention, a trunk circuit,

employed in establishing connections between an operators position andan automatic branch exchange, includes a pad control relay connected toa talking conductor for operation by ringing current when the ringingkey of a connected cord is operated, the impedance of the connectionfrom the talking conductor through the pad control relay beingsufiicient to prevent interference with voice current transmissionthrough the trunk circuit and to prevent interference with thetransmission of dial impulses. According to a feature of the invention,a pad control relay is connected through a high impedance device to oneof the talking conductors of a trunk circuit, the operation of the relayresponsive to the connection of a ringing current source thereto beingeffective to decrease the attenuation of the voice currents transmittedthrough the trunk circuit, to disconnect the relay irom the talkingconductor, and to close a holding circuit independent of the talkingconductor.

The invention and its features will be better understood by considering.a system in Which the invention is embodied, one such system being shownin the drawings which form a part of this specification. The inventionis, however, not limited in its application to the specific system shownbut is generally applicable to systems comprising means includingresistance pads for controlling the attenuation of a connection.

Referring to the drawings, Fig. 1 represents an automatic branchexchange comprising a subscribers station A, line LI and line circuitLCI, a line-finder LFI, a selector-connector SCI, an incoming connectorICI and a trunk circuit TCI; and 7 Figs. 2 and 3 and 4 show in a secondofiice a trunk circuit TCZ, jack ended at an operators position, atelephone repeater RI individual to the trunk circuit, and a cordcircuit CDI at the operator position.

The subscribers station A is of the usual common battery type andincludes a dial or other impulse sender for. controlling the selectiveoperation of the automatic switches through which desired connectionsare established. The line-finder LFI and selector-connector SCI andincoming connector ICI, may be of the usual two-motion step-by-steptype. Reference may be had to Automatic Telephony, 2nd Edition by Smithand Campbell, pages 53 to for description of the structure of suchswitches andtheir operation when used as selectors and connectors.Reference may be had to the patent to Stokely No. 1,799,654, grantedApril 7, 1931, for a description of a line-finder switch of thestepby-step two-motion type. Reference may be had to the patent to A. E.Hague No. 1,675,886, July 3,1928, for a description ofaselector-connector of the step-by-step two-motion type. The linefinderLFI and selector-connector SCI and incoming connector ICI are eachrepresented in the drawings by a set of brushes and either one or twosets of terminals, all other apparatus associated therewith beingomitted. The line LI is connected to terminals IS in the bank of finderLFI, to terminals I8 in the bank of selector-connector SCI and toterminals 20 in the bankof the incoming connector ICI. The line circuitLC'I, while not shown in detail, comprises the usual line and cut-offrelays. The trunk circuit TCI connected to terminals. I9 in the bank ofselector-connector SCI in the automatic branch ofiice is associated witha two-conductor trunk line TI extending to the second ofiioe. In thesecond ofiice, trunk TI is connected to a trunk circuit T02 having ajack at an operators position. The trunk circuits TCI and T02 are shownin detail in the drawings and their operations in establishingconnections in both directions over the trunk TI are hereinafterdescribed. The telephone repeater is not shown in detail since such ashowing is not necessary; and only so much of the operators cord CDI isshown as is necessary for a clear and complete explanation of applicantsinvention.

Assume now that a call is initiated at the subscribers station A, thatthe line circuit LCI and a line-finder LFI are operated to extend theline LI to a selector-connector SCI,- and that the selector SCI isoperated under the control of the dial at the calling station to selectthe terminals It! to which the trunk circuit TCI is connected. Uponseizure of the terminals I9, the selectonconnector SCI temporarilyconnects ground to brush I1 and therethrough to sleeveconductor O 23 oftrunk circuit 'ICl, thereby effecting the operation of relay 24. Relay24 locks through its inner lower front contact to conductor 23independent of relay 31, and connects conductor 23 to the winding ofrelay 26. With relays 24 and 25 operated, a circuit is closed foroperating relay 3!, this circuit being traced from battery through theupper winding of'relay 3!, upper continuity back contact and outer lowerback contact of relay 32, uppermost front contact of relay 26, lowerright winding of repeating coil 25, inner upper front contact of relay26, conductor 22, brush N3 of switch SCI, middle brush of finder switchLFl, line LI and station loop,

upperbrush of finder switch LFl, brush l of switch SCI, conductor 2|,outer upperfront contact of relay 26, upper right winding of repeatingcoil 25, inner upper front contact of relay 2'4, outer upper backcontact and lower continuity back contact of relay 32, and lower windingof relay St to ground. The-operation of relay 3| closes a circuit foroperating relay 38 and closes a-circuit foroperating relay 5| of thetrunk circuit T62 atthe distant end of trunk TI. The operation of relay38 oftrunk circuit TCI closes a-circuit for operating relay 31, andrelay 31 connects ground to sleeveconductor 23 to hold relays M -and 25operated and to holdthe selector connector SCi andlline finder LF!operated 'after the ground temporarily connected to brush ll of theselector-connector SCI is disconnected therefrom, The circuit foroperating relay-5| is traced from battery through the upperwinding-ofrelay-5l, l.ower back contact of relay 50,'1ower conductor oftrunk Tl and conductor- 42 of trunk circuit 'I'Cl, lowermost frontcontact-of relay 26, middle lower front contact of relay fil, outerlower continuity backcontact of relay-28, innerlower front contact ofrelay 26, lower left winding of repeating coil 25, middle of relay andthrough the lower winding of relayr5l to ground. Relay 36 is polarizedand the currentthrough itslower winding is in the non-operatingdirection so that relay 36 is not operatedat this time. The operation ofrelay 5| oft-trunk circuit TC2 closes a circuit for operating' relay 84.Theoperation of relay 84 connects ground to the sleeve of multiple jacks95 as albusy indication, closes a circuit for lighting the busy lamps91, and closes a circuit including the inner lower front contact ofrelay 5! foroperatively energizing the winding of relay 6 2. Theoperation of relay 62 closes a circuit including the lower backcontactof relay 82 "for lighting the-trunk lamp 96; and disconnects ground fromthe lower outer transfer spring of relay 53 to prevent the operation ofeitherof relays 54 and 58. 'At its upper front contact, relay 52 closesa circuit including the outer lower front contact of relay '5! foroperating-relay 50. The operation of relay 50 closes a locking circuitthrough its outer lower front contact to ground at the inner-lower frontcontact of relay 84, relay 62 beinglocked through its upperfront'contact to-the same ground. Relay 50 disconnectsthe conductors oftrunk Ti 4 from the windings of relay 5| and'connects these conductors,through front contacts of relay 62 and back contacts of relay 63, to thewindings of line relay 5T. Relay 5| is thereby released; and relay 5! isoperated in the circuit through the conductors of trunk Ti and thebridge across these conductors comprising the lower winding of relay 35of trunk circuit TCi. Relay 84 is held operated, after relay 5!releases, under the control of relay 51. With relays 50, 52 and 84operated, a connection is closed from the ringing current source 85,through condenser 81', lowermost front contact of relay 85, inner upperback contact of relay ,82, inner lower front contact of relay 52' andinner'lower front contact of relay 55, to the lower conductor of trunkTi, thereby to transmit an audible ringing tone to the callingsubscriber at station A.

The lighting of the trunk lamp 98 associated with jack 55 attracts anoperators attention and indicates that there is a call waiting to beanswered. Whenan operator answers-the call by inserting the plug ill ofa cord CD! in jack 95, relay [5% is operated by the closure of a circuitfrom ground at the inner upper front contact of relay 8%, sleeveconductors of jack 95 and plug iili, inner lower back contact of relayN3 of cord CDi and the winding of relay I06, to battery. The operationof relay I55 closes a circuit from battery through the lower winding ofretard coil I39, lower front contacts of relay I06, lower winding ofrelay I55, lower back contact of key H33, ring conductors of plug l8!and jack 95, conductor 92, windings of retard coil and relay 8!, a backcontact of relay 53, conductor 9!, tip conductors of jack and plug llll,upper back contact of key 593, upper winding of relay 55, upper frontcontacts of relay IE5, through the middle winding of retard coil ms toground. Relayfi i is thereby operated to close a circuit which includesthe upper front contact of jack 95 for operating relay 53; Relay 53looks to the ground at the contact of jack 95 independent of relay 8|,closes a circuit for holding relay 84, closes a circuit for holdingrelay 59, closes a circuit for operatively energizing the lower windingof relay 82, closes a circuitincluding the outer lower front contact ofrelay 5'! for operating relay 64, and interchanges the connectionsbetween the windings of relay E'I'and the conductors of trunk Tl so asto reverse the direction of current through the lower winding ofpolarized relay 36 of trunk circuit TC! and thereby cause'the operationof relay 35. The operationof relay 82 closes a looking circuit throughthe upper winding of this relay to battery at the lowermost frontcontact of relay 62, opens the circuit path for lighting the trunk lamps96, and opens the connection for transmitting an audible ringing signalto the calling station. The operation of relay 64 opens theshort-circuit normally connected across condoctors 65 and 65, whichconnect with the input side of one of the amplifiers of the telephonerepeater, and closes a short-circuit across resistor 68 and condenser69. The bridge, consisting of the winding of relay 8! and the windingsofretard coil 86, across tip and ring conductors of jack 95 is opened bythe operation of relay 63 and again closed by the operation of relay 54.The continued closure of this bridge effects the operation ofsupervisory relay I05 of the cord thereby extinguishing the rear cordsupervisory lamp I02. The aforementioned operation of relay 82 connectsthe winding of relay 52 to conductor 9!, whereby relay '52 isoperatively energized in series with the upper Winding'of relay I05 andmiddlewinding of retard coil I09.

The operation of relay. 52 closes a circuit for operating relay 53; andthe operation of relay 53 completes the talking'connection between the,

relay I05 of cord CDI, condensers, I01 and N33,.

back contacts of relay I I3, front contacts of talking key I I4, to theoperators telephone set. The aforementioned operation of relay 36oftrunk circuit TCI closes a circuit for operating relay 32 and-acircuit for operating relay 39. The operation of relay 32 interchangesthe connection between the windings of relay 3| and the conductors ofline LI thereby to reverse the current through the conductors of lineLI. Relay 39 looks under control of relays 31 and 38 and connects theupper high resistance winding of-relay 36 in parallel with its lowresistance winding.-

The answering operator may further extend the connection by plugging theother end of the cord into a jack of a called, extension, into the jackof another tie line or into the jack of a trunk to the central ofiice.

If the receiver hook at the calling station is operated to recall theoperator, relay 3| of trunk circuit TCI is alternately released andreoperated, thereby alternately opening and closing the connectionthrough the low resistance winding of relay 36. The operation of relay36 is maintained by its upper, high resistance winding, but relay 5'! oftrunk circuit TC2 releases and reoperates each time the low resistancebridge across the conductors of trunk TI is opened and closed by relay 3I. The release and reoperation of relay 5? cause the release andreoperation of relay 64, whereby the bridge across conductors 9| and 92is alternately opened and-closed to re-- lease and reoperate supervisoryrelay I05 of cord CDI and thereby flash the answering supervisory lampI92. If the telephone instrument is replaced at the calling station, torelease the connection, before the cord CDI is disconnected from jack95, relay 3| of trunk circuit TCI, relays 51 and 64 of trunk circuitT02, and relay I05 of cord CDI are released in succession, therebycausing the answering cord supervisory lamp I92 to be lighted as-adisconnect signal. The continued release of relay 3| causes the releaseof relay 38;

but relay 31 is held operated until the answering operator disconnects,the circuit for holdin relay 3'! being traced through the back contactof relay 38, a front contact of relay 39 and the front contact of relay35. When the operator disconnects plug IIlI from jack 95, relays 52 and63 release; the release of relay 52 causes the release of relay 53; therelease of relay 53 causes the release of relays 84, 62 and 59; and therelease of relay 62 causes the release of relay 82. The release of relay84 removes the busy indication from the sleeves of the trunk jacks95 andextinguishes the busy lamps 91. The aforementioned release of relay 53also interchanges the connections between the windings of relay 5! andthe conductors of trunk TI, thereby reversing the current through theseconductors to effect the release of relay 36 of trunk circuit TCI. Therelease of relay 36 causes the release of relays 32 and 31; and therelease of relay 31 causes the release of relays the busy lamps '91.

24, 26 and'39. whereby'the trunkcircuit TCI is again normal. 1

Assume next that an operator has answered a call with plug IIII of cordCDI, that the call is fora station in the dial branch exchange and thatthe operator inserts the plug I19 in jack to extend the connection overtrunk TI and that the talking and dialing key I Id of cord CDI is inoperated position. Relay 8| of trunk circuit T02 is thereby operated bythe current in a circuit from battery through the lower winding of relayI I I, outer lower back contact of relay I I3, a front contact oftalking key-I I4, conductor I54, a back contact of relay I5! of theoperators position,

conductor I52, a front contact of key H4, lower back contact of ringingkey II5, ring conductors of plug H9 and jack 95, conductor 92, windingsof retard coil 89 and relay 8|, back contact of back contact of relayI51 of the operators position, conductor I53, a front contact of keyIM', outer upper'back'contact of relay II3, upper winding'of relay I II, and inner upper back contact of relay II3, to ground. The operationof relayBI closes the circuit, including the upper front contact ofjack. 95, for operating relay 63. Relay 63 looks to ground at the upperfront contact of jack 95, opens the circuit for operating relay 8|, andcloses circuits for operating relays 8d, 59 and 82. The operation ofrelay 84 connects ground to th sleeve of jacks 95 and lights I conductor9|, tip conductors of jack 95 and plug II 9, upper back contact ofringing key II5, a front contact of talking and dialing key II 4,conductor I5I, upper back contact of relay I51 of the operatorsposition, conductor I53, another front contact of key II4, outer upperback contact of relay I I3, upper winding of relay I II and inner upperback contact of relay H3 to ground. The resistance of relay 52 is highso that the current in this circuit effects the operation of relay 52without effecting the operation of relay III. The operation of relay 52closes a circuit for operating relay 53. The operation of relay 53closesa circuit for operating relay 54, connects the conductors of trunk TI toconductors 55 and59 leading to the telephone repeater, and also bridgesthe winding of retard coil 60 and winding of relay 6| in series acrossthese conductors to cause the operation of relay 3| of trunk circuit TCIin the dial private branch exchange. The circuit for operating relay 3|is traced from battery through its upper winding, upper continuity backcontact of relay 32, outer'lower back contact of relay 32, middle lowerback contact of relay 26, lower left winding of repeating coil 25, innerlower back contact of relay 2-6, conductor 42, lower conductor of trunkTI, inner lower front contact of relay 50, inner upper back contact ofrelay 62, lower continuity back contact of relay ,58, winding of retardcoil 69, winding of relay 6|, upper continuity back contact of relay 58,upper front contact of relay 53, upper front The operation of relay 50upperiback' contact of relay, outerrupp'er back." contact of relay 32,lower COIltiIIUitYZbaCkT con= tact ofzrelay 32; and lowers-winding ofrelay3I,

to'ground. The oper'ationofirelay 3.1 causes'the" successive operations.i'of relays .238" 3:1, the operation of-relay 3'! connecting. ground toconductor 23 to mark the trunk circuit TCI busy in the banks ofselector-connectors havingaccess' thereto. The operation of relay 3|also closes a circuit for operating the line relay 45* ,of the incomingconnector ICI; Thiscircuit is traced from battery through the rightwinding of relay 65, right back contact; of'relay II, conductor '43,

inner lower back contact of relayz i, inner front contact of relay. 3!,innerlower back contact of relayZB, outer upper back contact of relay24,

upperright winding of repeating coil 25, middle upper back contact ofrelay 24, upper back contact ofrelay 28, lower winding of relay 36,outer. upper back contact of relay 25, lower right winding of repeatingcoil 25, inner upper back contact of relay 26, outer lower back contactof relay28,

middle lower front contact of relay 3|, lowermost back contact of relay25, conductor, left back contact of relay 41, and left winding of relay,

to ground. The incoming connector ICI is thereby prepared for responseto dial impulses.

The aforementioned operation of relay 5c closes circuits for maintainingthe operation of relays 84 and 59, and closes a circuit from ground atthe inner upper back contact of relay. 64, through a front contact ofrelay 5B, resistor fil, conductor 92, ring conductors of jack 95 andplug H8, lower back contact of ringing key H5, a front contact of keyIM, conductor I52, lower back contact of relay 551 of the operatorsposition, conductor I55, another front contact of key H4, lowermost backcontact of relay H3 and through the lower winding of relay II I tobattery. Resistor 61 limits the current to prevent operation offrelayfII I. When the operator dials each of the digits of the number of thecalled station in the dial branch exchange, the ofi-normal contacts ofdial I58 are actuated to eifect the opera-,

tion of relay I55 and to eiiect the operation of relay I51, the circuitfor operating 'relay I57 not being shown. With relays I56 and I5!operated, the windings of relay I I I are disconnected from the'tip andring conductors of plug II9so as to place relay :32 under the control ofthe impulse contacts of the dial E]; The circuit for operating relay 52is now traced from battery through the winding of relay 52; outer upperfront contact of relay 82, conductor BI, tip conductors'of jack 95andplug I19, upper back contact of ringing key H5, a front contact oftalking and dialing key H4, conductor I5I, impulse contacts of dial I56,front contact of relay I56, conductor I52, a front contact of key II I,aback contact of key H5, ring conductors of plug IIS and jack 95,conductor 92, resistor 81, a front contact of relay 5 and a back contactof .relay 64, to ground. Relay 52 is alternately released and reoperatedresponsive to each impulse cre- .ated by the operation of dial I50, thealternate release and reoperation of relay52 causing. a like release andreoperation of relay 53. The release of relay 53 in response to thefirst dial impulse of each digit closes a circuit for operating relay50; 53 -andziit; a'ndrthe circuitfor energizing'the lower winding ofrelay 3| includes'the upper conductor of trunk TI and upper frontcontacts of relays 50; 53 and'58; Relay58 is slow-to-release andremainsoperateduntilrelay'53 has reoperatedafter the lastiirnpulseof atrain; Relay SI of trunk circuit TCI 'is thereby released'and reoperatedin response to 'the dial impulses created by' the dialing ofv the digitsof the called'number in the dia1 private branch exchange with whichconnection isxdesired', The release of relay 3I, in response to thefirst dial impulse of each digit, opensthe bridge acrossv conductorsHand to release the "line relayL IE of connector 'ICI and closes acircuit from" ground at its lower back contact, through aback contact ofrelay-39 and a front contact ofrelay-38, for operating relay 28.Relay'28 is a slow-to-release relay which remains operated during thereceiptof each train of impulses by relay 3i. Whenrelay 3| reoperatesat'the end of the firstdial impulse, the windings'of line relay 45 oftheconnector are energized'in separate groundedcircuits, The circuitfor-energizing the right Winding of'relay 45 includes conductor 43,inner'front contact of relay 3i and inner lower front contact of relay28; and the circuit for energizing the left winding of relay 45'includes conductor 44, the middle lower front contact of relay SI andthe outer lower front contact of relay 28. The release and reoperationof relay 3I responsive to the succeeding dial impulses ineach traincauses the alternate release and' reoperation of relay 45. At the end'of each of these trains of dial impulses relays 31 and 45' remainoperated and relay 28 releases to connect the lower winding of relay36in series with the windings of "relay 45; The response" of relay 45 tothe dialing of the digits of the number of the calledstation operatesconnector ICI to select and test the terminals connected'to thecalledline; and," if the line is idle, ringing current is transmitted to thecalled station. When the called subscriber. answers, relay 3'! isoperated, therebyefiectinga reversal of the current through the lowerwinding of' relay 36 to operatively-energize this winding. The operationof relay 36 closes a circuit for operating relay 3'2 and a, circuit foroperating relay 39.

Relay 39 looks under'control of relays 31 and 38 and connects the upperwinding of relay 36 in parallelwith its lower winding. The operation ofrelay 32 interchanges the connections between the windings of relay 3iand the conductors of trunk TI thereby to reverse the currentrthrougrhthe winding of relay EI of trunk circuit TCZ to cause the operation ofrelay 6i, and relay GI.

closes. a circuit for. operating relay 64. The, operation of relayshort-circuits resistor 68 and condenserfiil; opens the short circuitacross conductors 65: and 65,- closes the talking circuit, opens theconnection of conductor 92 to ground through resistor El 'toimprove thetransmission balance, and connects thewinding of relay SI and retardcoil 881' across the tip and ring conductors of the jack 95 and plugH9." When the talking and dialing key'of cord CDI is restoredto normal,relay 52 is held operated in series with the upper wind ing of relayI II andrelay SI is operated in series to extinguish the" supervisory lampIIB'. Relay" 8| is operated but performs no useful function atthistime.

If the subscriber at the called station alternately restores and removesthe telephone, relay 4'! is alternately released and reoperated, therebycausing the alternate release and reoperation of relay 35. Relay 99remains operated, but relay is alternately released and reoperated toreverse the current over the conductors of trunk TI, thereby effectingthe alternate release and reoperation of relay 5| of trunk circuit T02.The alternate release and reoperation of relay 6| causes the alternaterelease and reoperation of relay 64, thereby to open and close thebridge across conductors 9I and 92, relay 52 being held operated duringflashing in series with the upper winding of relay I II. Theenergization of the upper winding of relay I I I in series with relay 52and the energization of the lower winding of relay I I I in series withresistor 6'! are ineffective to hold relay III operated and relay III isalternately released and reoperated under the control of relay 64, toflash supervisory lamp H8 in the cord circuit. When the telephone isrestored to normal at the called station, relay 4'! releases and thereversal of the current through conductors 43 and 44 causes the releaseof relays 36 and 32 of trunk circuit TCI. The release of relay 32reverses the current in the conductors of trunk TI to cause thesuccessive release of relays BI and B4 of trunk circuit T02. The releaseof relay 64 opens the bridge across the'conductors 9i and 92 to causethe release of relay III to light the supervisory lamp H8. Relay 52remains operated to ground in the cord circuit until the cord isdisconnected from jack 95. When the operator disconnects plug I I9 fromjack 95, relays 63, 52 and 8I release. The release of relay 63 causesthe release of relay 82; and the release of relay 52 causes the releaseof relay 53. The release of relay 53 causes the release of relay 54 andopens the bridge across conductors of trunk TI thereby to release relay3i of trunk circuit TCI. The release of relay 54 also causes the releaseof relay 84, whereby the busy indication is disconnected from the sleeveconductor of jacks 95 and the busy lamps 9'! are deenergized; and causesthe release of relay 59 whereby the trunk circuit T02 is restored tonormal. The release of relay 3| of trunk circuit TCI causes thesuccessive release of relays 38, 31 and 39 and causes the release ofrelay 4'! of connector ICI. The release of relay 3? disconnects groundfrom conductor 23 to terminate the busy indication in the banks of theselectors having access to the trunk circuit TCI. The release of relay41 causes the release of relay 46 and the connector is restored tonormal in usual manner.

If on a call initiated by the operator to a called subscriber in thedial branch exchange, a busy condition is encountered, busy tone fromthe connector is transmittedthrough repeating coil 25, trunk TI andtrunk circuit T02 to the calling operator. Since the called station has.not answered, relays GI and 64 are not operated so that the talkingconnection is open at the lower front contact of relay 64; but thecondenser 69 and resistor 68 constitute a path suitable for thetransmission of the busy tone from the conducthe dial private branchexchange is a toll call re- 10 quiringhigher grade transmission, or if*the' above described call to astation in the dialp'ri vate branchexchange is of such a character, the Dad control III relay of'trunk'circuit T02 is operated by application of ringing voltage toconductorsBI and 92. If plug I M is-connected' toc jack ringing key I03 isoperated and if plug H9 is connected to jack 95 ringing'key H5 is isbridged across conductors 9| and 92 and'th'e voltage applied toconductor 92, through the elements of gas-filled tube 83 and the windingof relay I0 causes the break-down of the tube and u the operativeenergiz'ation of the relay.- Relay 1 19 looks through conductor 94 toground'at the upper contactof jack 95 as long asthe' plug of the cordconnected to jack 95 at the same-time opening the connection throughtube'83 to conductor 92. The operation of relay III short; i cuits theseries resistors of pad PI and opens the bridge across these resistorsto decrease the attenuation of the talking currents. While the specificarrangement showns'a gas-filled tube as being the high impedance elementnormally-con necting the winding of relay III to the talking conductor92, a thermistor may be used-in place of the tube. What is claimed is:

1. In combination in a telephone system, a' trunk circuit including anattenuation pad comprising resistors normally connected inseries. withthe talking conductors of said'trunk cir-H cuit, an operators cordconnected, to said trunk circuit, a'pad control relay for shortcircuiting' -said resistors, means including a normally high impedanceelement connecting said relay to one of said talking conductors,a'direct current source connected to the talking conductors of saidcord;

a ringing .current source, and a ringing key in said cord for connectingsaid ringing current source to the talking conductors of said cord inplace of said direct currentsourcefthe voltage" of said direct currentsource being insumci'ent and the voltage of said ringing current sourcebeing suflicient to effect the operative energiz'ation of the winding ofsaid'relay.

of .said direct current source, "the voltage of said direct currentsource being insuflicient and the voltage of said ringing currentsource'being sufficient to effect the operative energization of thewinding of said relay. contacts actuated bythe operative energizationofthe Winding of saidre-1 lay for disconnecting the winding fromsaid'one talking conductor, and means for locking said relay operated aslong as said cord is connected to said trunk circuit.

3. In combination in a telephone system, a trunk circuit including anattenuation'pad com-' prising resistors normally connected in seriesWith the talking conductors. of'sai'd trunk circuit, an operator's cordconnected to said-trunk 'CllClll'G,

a pad'control relay for short circuiting said-ire: 1;

sisters, means including-a normally high impedance elementconnecting-saidqrelay toone of said talkingz condu'ctors, a directcurrent source connected 'tothe talking conductors of said cord, aringing current source, a ringingkey in said cord forconnecting saidringing current source tomthe talkingconductors of. said cord in placeof said directcurrent source,the voltage ofisaid directcurrentsourcebeing insufficient and the voltage of said ringing current source being,sufficientto effect the operative energization of the winding: ofsaidrelayrand means. rendered effectiveby the operation of said relay formaintaining 'thefshort-zcircuit across the resistors of saidpadswhileasaid cordis; connected to said trunk-:circuit;

475 11131 21 telephone system comprising subscribers lines; trunk-lines,andanoperators link fonuse in-establishing-rconnections between saidlines, said trunkin'cluding a source of direct current, a: source ofringingcurrent and means for; connecting thesource' ofringing current toa connectedvline-in: place. of the source of direct current; a-trunkicircuit individually terminating one of :said, trunk. lines, an:amplifying repeater insthe' talkingconnectionthrough said: trunkcircuit, an attenuation pad connected in series with the talkingconductors of said -trunk circuit, aerelay in said trunk circuit, avariable impedance element-connecting the winding of said relay to: a:talking conductor of said trunk circuit, saidelement" normally having ahigh impedance, theconn'ection'ofthez'direct current source in said linkto the talking conductors of said trunk circuit being ineffective'todecrease the impedance of saidele'ment and the connection of the sourceof ringing current in said link to the talkingcon'ductors ofsaid trunkcircuit being effective to decrease the impedance of said element 'to'effect the 'operationof said relay, and means includingcontacts' of saidrelay forrendering said pad'inefiective to. attenuate currenttransmitted throughithe talking conductors of' said trunk circuit;

In a telephone system comprising subscribers lines, trunk lines, and anoperators link for usein establishing connections between saidlines,'.said link including a source of direct cur rent, a sourceof'ringing current and means for connecting: thesource of ringingcurrent to a connected line inplace of thesource of direct current;,atrunk circuit individually terminating one =ofsaid trunk lines, anamplifying repeater in the :talking :connection through said'trunkcircuit, an attenuation pad. connected in series with thettalk'ingconductors of said trunk circuit, a relay in said trunkcircuit, a variable impedance element connecting the Winding of saidrelay to a talking conductor of said trunk circuit, said elementnormally having a highimpedance, the connection of the direct currentsource in said link to the talking conductors of said trunk circuitbeing ineffective to decrease'the impedance of said element and theconnection of the source of ringingcurrent in said link to the talkingcon- ,ductors of' said trunk circuit'being effective to decreasetheimpedance of said element to effect the. operation of said'relay, andmeans including contacts of said relay for rendering said padineffective to attenuate current transmitted through the-talkingconductors of said trunk circuit and for disconnecting said relay fromthe talking conductor of said trunk circuit.

6. InaItelephone' system comprising subscribers 'li nes, trunk lines,and an operators link for 1'2 use in establishing connections betweensaid lines, said link including a source of direct current, a source ofringing current and means for connectingithe source of ringing currentto a connected line in place ofthe source of direct current, a trunkcircuit'individually terminating one of said trunk lines, an amplifyingrepeater in the talking'connectionthrough said trunk circuit, anattenuation pad connected in series with the talkingconductors of saidtrunk circuit, a relay in said trunk circuit, a variable impedanceelement connecting the Winding of said relay to a talking conductor ofsaid trunk circuit, said element normally having a high impedance, theconnection of the direct current source in said link tov thetalkingconductors of said trunk circuit being in.- eifective to decreasethe impedance of said element and-the connection of the source ofringing current in said link to the talking conductors ofsaid trunkcircuit being effective to decrease the impedance'of said element toeffect the operation of said relay, means including contacts of saidrelay for rendering said. pad ineffective to attenuate-currenttransmitted through'the talkingconductor of said trunk circuit, andmeans for locking said relay operated as long as said link is connectedto' said trunk circuit.

7. In a telephone system, a trunk circuit, an amplifying. repeater invthe talking connection through said trunk circuit, an attenuation padincludedin said talking connection, a relay in said trunk circuit, agas-filled tube connecting the winding. of said-relay to a talkingconductor of saidtrunk circuit, a source of direct current connected tosaid talking conductor during the existence ofa talking connectionthrough said trunk circuit, the connection of said direct current sourceto said talking conductor being inefiective to energize said tube; aringing current source and means for connectingsaid ringing source tosaid conductor in place of-said direct current source to effect theenergization of said tube and operation of said relay, andmeansincluding contacts of saidrelay for rendering said pad ineffectiveto. attenuate; current transmitted through said talking: connection.

8-. In a telephone system,. a trunk circuit. an amplifying repeater: inthe talking connection through said trunk circuit, an attenuation padincluded in said talking connection, a relay in said. trunk circuit, a.gas-filledtube connecting the winding of said relay to a talkingconductor of said trunk circuit, the impedance of said tube being highenough, to prevent interference with thetransmission oftalking' currentthrough the trunkcircuit', to prevent interference with the transmissionof dial impulses and to prevent the operation of said relay Whiletalking current is being transmitted through the trunk circuit, vmeansfor applying a ringing voltage to said talking conductor to effect thebreakdown of the tube and operation of said relay, and means includingcontacts'of said relay for disconnecting the winding of said relay fromsaid tube and talking. conductor. and for rendering said pad ineffectiveto attenuate current transmitted through said talking connection.

9. In a telephone system, a trunk circuit, an amplifying repeater in thetalking connection .through said trunk circuit, an attenuation padincluded in said talking connection, a relay in said trunk circuit, agas-filled tube connecting the winding of said relay to a talkingconductor of said trunk circuit, the impedance of said tube being highenough to prevent interference with the transmission of talking currentthrough the trunk circuit, to prevent interference with the transmissionof dial impulses and to prevent the operation of said relay whiletalking current is being transmitted through the trunk circuit, meansfor applying a ringing voltage t said t 1 ing conductor to effect thebreakdown of the tube and operation of said relay, means includingcontacts of said relay for rendering said pad inefiective to attenuatecurrent transmitted through said talking connection, and means forlocking said relay while said talking connection is maintained.

10. In a telephone system, a trunk circuit, an amplifying repeater inthe talking connection through said trunk circuit, an attenuation padincluded in said talking connection, a relay in said trunk circuit, agas-filled tube connecting the winding of said relay to a talkingconductor of said trunk circuit, the impedance of said tube being highenough to prevent interference with the transmission of talking currentthrough the trunk circuit, to prevent interference with the transmissionof dial impulses and to prevent the operation of said relay whiletalking current is being transmitted through the trunk circuit, meansfor applying a ringing voltage to said talking conductor to effect thebreakdown of the tube and the operation of said relay, means includingcontacts of said relay for rendering said pad ineffective to attenuatecurrent transmitted through said talking connection and fordisconnecting the Winding of said relay from said tube and talkingconductor, and means for locking said relay while said talkingconnection is maintained.

PAUL L. WRIGHT.

